The GMOS On-Instrument Wavefront Sensor (OIWFS) consists of a 2x2 lenslet array on a moveable arm that patrols an area in front of the mask plane. Signals from the OIWFS are fed to the secondary for fast tip-tilt guiding.

• Filter : GG495 (longpass filter with a blue cut-off at 495nm)

• Limiting magnitude and availability of guide stars Guide stars for the OIWFS need to be fainter than V=9.5mag. Some limiting magnitude measurements for the OIWFS have been carried out during the commissioning; more are still planned. The table below gives the overview of the limiting magnitudes in photometric conditions and with low wind. The results are given as the limiting magnitude for a given observing condition and guide frequency. For programs that can be carried out in non-photometric conditions, the guide stars need to be sufficiently bright to enable guiding through the clouds. The limiting magnitude for the the OIWFS also depends on the seeing and the wind. In moderate to high wind the guide frequency needs to be at least 100Hz, while 50Hz is useful in low wind, only. Further, high wind makes the intial acquisition of a faint guide star a time consuming task, adding to the effective overhead at the time of observation.

  For the purpose of planning observations for the current semester, it is required that the guide stars be brighter than V=16 mag for guiding on dark sky in seeing conditions of 70-percentile or better. For guiding in moon light under the same seeing conditions, guide stars need to be brighter than V=15.0 mag. For programs that can executed in poorer seeing conditions, guide stars that are 1-2 magnitudes brighter are needed. Users are encouraged to select guide stars as bright as possible.

  OIWFS limiting (V) magnitudes
  Seeing	Dark sky	Full moon
  < 0.75arcsec	200Hz   15.5 mag 100Hz   16.2 mag   50Hz   17.1 mag	200Hz   15.0 mag 100Hz   15.3 mag   50Hz   15.5 mag
  1.5arcsec	200Hz   13.8 mag 100Hz   14.9 mag   50Hz   15.5 mag	Estimate 100Hz   13.0 mag

  
  
  

• Patrol field : 3.54 arcmin x 4.15 arcmin = = 14.7 sq arcmin, overlapping one corner of the imaging field with some area outside the imaging field. The patrol area is the red box on the schematic diagram below. The patrol area is effectively doubled since the field can be rotated 180 degrees. The OIWFS cannot patrol outside a circle with radius 4.8 arcmin; the green circle on the schematic diagram below. Thus a small area in the lower right of the patrol area shown on the schematic cannot be reached.
  Schematic diagram of the patrol area of the OIWFS (red box) relative to the GMOS CCD array (which consists of three chips). The imaging field is shown by the dashed line. The OIWFS cannot patrol outside a circle with radius 4.8 arcmin (the green circle).

• Vignetting of GMOS science field by OIWFS : An image showing the vignetting of the GMOS science field by the OIWFS is shown below. The OIWFS on that image is in the center of the field, just for illustration. The OIWFS can be placed anywhere in the patrol field outlined on the diagram above, including outside the imaging field.
  

  Vignetting of GMOS science field by OIWFS, if the OIWFS is in the center of the imaging field. The imaging field of view of 5.5 arcmin is shown. The width of the OIWFS arm as seen by the GMOS camera is about 20 arcsec.

In original form; Isobel Hook
Last update August 27, 2003; Kathy Roth